Plant stem cells: divergent pathways and common themes in shoots and roots

Plant stem cells and their applications: special emphasis on their marketed products

Stem cells are becoming increasingly popular in public lexicon owing to their prospective applications in the biomedical and therapeutic domains. Extensive research has found various independent stem cell systems fulfilling specific needs of plant development. Plant stem cells are innately undifferentiated cells present in the plant's meristematic tissues. Such cells have various commercial uses, wherein cosmetic manufacture involving stem cell derivatives is the most promising field at present. Scientific evidence suggests anti-oxidant and anti-inflammatory properties possessed by various plants such as grapes (Vitis vinifera), lilacs (Syringa vulgaris), Swiss apples (Uttwiler spatlauber) etc. are of great importance in terms of cosmetic applications of plant stem cells. There are widespread uses of plant stem cells and their extracts. The products so formulated have a varied range of applications which included skin whitening, de-tanning, moisturizing, cleansing etc. Despite all the promising developments, the domain of plant stem cells remains hugely unexplored. This article presents an overview of the current scenario of plant stem cells and their applications in humans.

Shikimic acid, a mannose bioisostere, promotes hair growth with the induction of anagen hair cycle

Shikimic acid (SA) has recently been found to be a major component of plant stem cells. The exact effects of SA on human hair follicles (HFs) is unknown. The purpose of this study was to examine the effects of SA on hair growth. We investigated the effect of SA on an in vivo C57BL/6 mouse model. We examined the expression of mannose receptor (MR), which is a known receptor of SA, in human HFs and the effect of SA on human dermal papilla cells (hDPCs), outer root sheath cells (hORSCs), and on ex vivo human hair organ culture. SA significantly prolonged anagen hair growth in the in vivo mouse model. We confirmed expression of the MR in human HFs, and that SA increased the proliferation of hDPCs and hORSCs. It was found that SA enhanced hair shaft elongation in an ex vivo human hair organ culture. SA treatment of hDPCs led to increased c-myc, hepatocyte growth factor, keratinocyte growth factor and vascular endothelial growth factor levels and upregulation of p38 MAPK and cAMP response element-binding protein levels. Our results show that SA promotes hair growth and may serve as a new therapeutic agent in the treatment of alopecia.

The DELLA Proteins Influence the Expression of Cytokinin Biosynthesis and Response Genes During Nodulation

The key event that initiates nodule organogenesis is the perception of bacterial signal molecules, the Nod factors, triggering a complex of responses in epidermal and cortical cells of the root. The Nod factor signaling pathway interacts with plant hormones, including cytokinins and gibberellins. Activation of cytokinin signaling through the homeodomain-containing transcription factors KNOX is essential for nodule formation. The main regulators of gibberellin signaling, the DELLA proteins are also involved in regulation of nodule formation. However, the interaction between the cytokinin and gibberellin signaling pathways is not fully understood. Here, we show in Pisum sativum L. that the DELLA proteins can activate the expression of KNOX and BELL transcription factors involved in regulation of cytokinin metabolic and response genes. Consistently, pea la cry-s (della1 della2) mutant showed reduced ability to upregulate expression of some cytokinin metabolic genes during nodulation. Our results suggest that DELLA proteins may regulate cytokinin metabolism upon nodulation.

Review: Chromatin organization in plant and animal stem cell maintenance

Stem cells have self-renewal capacity and can differentiate into specialized cell types. Although the origin, form and differentiated destinations of stem cells differ between animals and plants, they are regulated by similar epigenetic mechanisms during differentiation. There is increasing evidence that the three-dimensional (3D) genome organization plays important roles in gene expression regulation during stem cell differentiation. In plant cells, however, studies related to chromatin interaction in gene expression regulation are just beginning and will be a hot topic in the future. In this review, we summarized the similarities of plant and animal stem cell niches and their function in stem cell maintenance, the roles of chromatin conformation changes in regulating gene expression and recent findings about chromatin organization in plant cells at genome-wide and loci-specific levels.

To regenerate or not to regenerate: factors that drive plant regeneration

Plants have a remarkable regenerative capacity, but it varies widely among species and tissue types. Whether plant cells/tissues initiate regeneration largely depends on the extent to which they are constrained to their original tissue fate. Once cells start the regeneration program, they acquire a new fate, form meristems, and develop into organs. During these processes, the cells must continuously overcome various barriers to the progression of the regeneration program until the organ (or whole plant) is complete. Recent studies have revealed key factors and signals affecting cell fate during plant regeneration. Here, we review recent research on: (i) environmental signal inputs and physical stimuli that act as initial triggers of regeneration; (ii) epigenetic and transcriptional cellular responses to those triggers leading to cellular reprograming; and (iii) molecules that direct the formation and development of the new stem cell niche. We also discuss differences and similarities between regeneration and normal development.

MtNRLK1, a CLAVATA1-like leucine-rich repeat receptor-like kinase upregulated during nodulation in Medicago truncatula

Peptides are signaling molecules regulating various aspects of plant development, including the balance between cell division and differentiation in different meristems. Among those, CLAVATA3/Embryo Surrounding Region-related (CLE-ESR) peptide activity depends on leucine-rich-repeat receptor-like-kinases (LRR-RLK) belonging to the subclass XI. In legume plants, such as the Medicago truncatula model, specific CLE peptides were shown to regulate root symbiotic nodulation depending on the LRR-RLK SUNN (Super Numeric Nodules). Amongst the ten M. truncatula LRR-RLK most closely related to SUNN, only one showed a nodule-induced expression, and was so-called MtNRLK1 (Nodule-induced Receptor-Like Kinase 1). MtNRLK1 expression is associated to root and nodule vasculature as well as to the proximal meristem and rhizobial infection zone in the nodule apex. Except for the root vasculature, the MtNRLK1 symbiotic expression pattern is different than the one of MtSUNN. Functional analyses either based on RNA interference, insertional mutagenesis, and overexpression of MtNRLK1 however failed to identify a significant nodulation phenotype, either regarding the number, size, organization or nitrogen fixation capacity of the symbiotic organs formed.

Plant stem cells in cosmetics: current trends and future directions

Plant regeneration at the cellular and tissue level is a unique process. Similar to animals, the stem cells in plants have properties that help stimulate and regenerate plants after injury. The unique properties of plant stem cells have been a recent area of interest and focus both in developing new cosmetics and studying how these extracts/phytohormones will influence animal skin. This special report focuses on the current evidence-based trends in plant stem cell-based cosmetics and sheds light on the challenges that we need to overcome in order to see meaningful changes in human skin using topical cosmetics derived from plant stem cells.

A new wave of cosmetic ingredients containing plant stem cells and their extracts has made its way into the industry. What role do these ingredients play in affecting the aging skin? Several ancient practices such as Ayurveda have used plants as a mainstay of treatment for thousands of years. Plant stem cells could hold an interesting role if we can harness these benefits in cosmetics to create safe and effective organic topical skin care.

pRb-E2F signaling in life of mesenchymal stem cells: Cell cycle, cell fate, and cell differentiation

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into various mesodermal lines forming fat, muscle, bone, and other lineages of connective tissue. MSCs possess plasticity and under special metabolic conditions may transform into cells of unusual phenotypes originating from ecto- and endoderm. After transplantation, MSCs release the humoral factors promoting regeneration of the damaged tissue. During last five years, the numbers of registered clinical trials of MSCs have increased about 10 folds. This gives evidence that MSCs present a new promising resource for cell therapy of the most dangerous diseases. The efficacy of the MSCs therapy is limited by low possibilities to regulate their conversion into cells of damaged tissues that is implemented by the pRb-E2F signaling. The widely accepted viewpoint addresses pRb as ubiquitous regulator of cell cycle and tumor suppressor. However, current publications suggest that basic function of the pRb-E2F signaling in development is to regulate cell fate and differentiation. Through facultative and constitutive chromatin modifications, pRb-E2F signaling promotes transient and stable cells quiescence, cell fate choice to differentiate, to senesce, or to die. Loss of pRb is associated with cancer cell fate. pRb regulates cell fate by retaining quiescence of one cell population in favor of commitment of another or by suppression of genes of different cell phenotype. pRb is the founder member of the "pocket protein" family possessing functional redundancy. Critical increase in the efficacy of the MSCs based cell therapy will depend on precise understanding of various aspects of the pRb-E2F signaling.

Convergence of stem cell behaviors and genetic regulation between animals and plants: insights from the Arabidopsis thaliana stomatal lineage

Plants and animals are two successful, but vastly different, forms of complex multicellular life. In the 1600 million years since they shared a common unicellular ancestor, representatives of these kingdoms have had ample time to devise unique strategies for building and maintaining themselves, yet they have both developed self-renewing stem cell populations. Using the cellular behaviors and the genetic control of stomatal lineage of Arabidopsis as a focal point, we find current data suggests convergence of stem cell regulation at developmental and molecular levels. Comparative studies between evolutionary distant groups, therefore, have the power to reveal the logic behind stem cell behaviors and benefit both human regenerative medicine and plant biomass production.

Anatomical aspects of angiosperm root evolution

BACKGROUND AND AIMS

Anatomy had been one of the foundations in our understanding of plant evolutionary trends and, although recent evo-devo concepts are mostly based on molecular genetics, classical structural information remains useful as ever. Of the various plant organs, the roots have been the least studied, primarily because of the difficulty in obtaining materials, particularly from large woody species. Therefore, this review aims to provide an overview of the information that has accumulated on the anatomy of angiosperm roots and to present possible evolutionary trends between representatives of the major angiosperm clades.

SCOPE

This review covers an overview of the various aspects of the evolutionary origin of the root. The results and discussion focus on angiosperm root anatomy and evolution covering representatives from basal angiosperms, magnoliids, monocots and eudicots. We use information from the literature as well as new data from our own research.

KEY FINDINGS

The organization of the root apical meristem (RAM) of Nymphaeales allows for the ground meristem and protoderm to be derived from the same group of initials, similar to those of the monocots, whereas in Amborellales, magnoliids and eudicots, it is their protoderm and lateral rootcap which are derived from the same group of initials. Most members of Nymphaeales are similar to monocots in having ephemeral primary roots and so adventitious roots predominate, whereas Amborellales, Austrobaileyales, magnoliids and eudicots are generally characterized by having primary roots that give rise to a taproot system. Nymphaeales and monocots often have polyarch (heptarch or more) steles, whereas the rest of the basal angiosperms, magnoliids and eudicots usually have diarch to hexarch steles.

CONCLUSIONS

Angiosperms exhibit highly varied structural patterns in RAM organization; cortex, epidermis and rootcap origins; and stele patterns. Generally, however, Amborellales, magnoliids and, possibly, Austrobaileyales are more similar to eudicots, and the Nymphaeales are strongly structurally associated with the monocots, especially the Acorales.

Structural and component characterization of meristem cells in Araucaria angustifolia (Bert.) O. Kuntze zygotic embryo

Araucaria angustifolia, the Brazilian pine, is an endangered native conifer with economic and ecological importance. The female cone develops seeds containing the zygotic embryo, which, at cotyledonary stage, shows well-developed meristems. Little is known about the structure of gymnosperm meristems. In the present work, the composition and morphological organization of Araucaria angustifolia shoot and root apical meristems were studied during embryo development, using histochemical and microscope analyses. Histochemical evaluation revealed the presence of cellulose within the cell wall, cells with the presence of total proteins that react with Coomassie Brilliant Blue, starch grains, and large nuclei with evident nucleoli in the cytoplasm. Scanning electron microscopy showed apical meristem surface morphology, and both scanning and transmission microscopy revealed a thin and irregular cell wall with plasmodesmata and within the cells, mitochondria, many vacuoles, lipid bodies, Golgi bodies, and many amyloplasts with endoplasmic reticulum surrounding them and large nuclei. Similar to angiosperm cells, A. angustifolia meristem cells exhibit pluripotent characteristics, such as apparatus for intercellular communication and differentiation.

TALE and Shape: How to Make a Leaf Different

The Three Amino acid Loop Extension (TALE) proteins constitute an ancestral superclass of homeodomain transcription factors conserved in animals, plants and fungi. In plants they comprise two classes, KNOTTED1-LIKE homeobox (KNOX) and BEL1-like homeobox (BLH or BELL, hereafter referred to as BLH), which are involved in shoot apical meristem (SAM) function, as well as in the determination and morphological development of leaves, stems and inflorescences. Selective protein-protein interactions between KNOXs and BLHs affect heterodimer subcellular localization and target affinity. KNOXs exert their roles by maintaining a proper balance between undifferentiated and differentiated cell state through the modulation of multiple hormonal pathways. A pivotal function of KNOX in evolutionary diversification of leaf morphology has been assessed. In the SAM of both simple- and compound-leafed seed species, downregulation of most class 1 KNOX (KNOX1) genes marks the sites of leaf primordia initiation. However, KNOX1 expression is re-established during leaf primordia development of compound-leafed species to maintain transient indeterminacy and morphogenetic activity at the leaf margins. Despite the increasing knowledge available about KNOX1 protein function in plant development, a comprehensive view on their downstream effectors remains elusive. This review highlights the role of TALE proteins in leaf initiation and morphological plasticity with a focus on recent advances in the identification of downstream target genes and pathways.

TALE and Shape: How to Make a Leaf Different

The Three Amino acid Loop Extension (TALE) proteins constitute an ancestral superclass of homeodomain transcription factors conserved in animals, plants and fungi. In plants they comprise two classes, KNOTTED1-LIKE homeobox (KNOX) and BEL1-like homeobox (BLH or BELL, hereafter referred to as BLH), which are involved in shoot apical meristem (SAM) function, as well as in the determination and morphological development of leaves, stems and inflorescences. Selective protein-protein interactions between KNOXs and BLHs affect heterodimer subcellular localization and target affinity. KNOXs exert their roles by maintaining a proper balance between undifferentiated and differentiated cell state through the modulation of multiple hormonal pathways. A pivotal function of KNOX in evolutionary diversification of leaf morphology has been assessed. In the SAM of both simple- and compound-leafed seed species, downregulation of most class 1 KNOX (KNOX1) genes marks the sites of leaf primordia initiation. However, KNOX1 expression is re-established during leaf primordia development of compound-leafed species to maintain transient indeterminacy and morphogenetic activity at the leaf margins. Despite the increasing knowledge available about KNOX1 protein function in plant development, a comprehensive view on their downstream effectors remains elusive. This review highlights the role of TALE proteins in leaf initiation and morphological plasticity with a focus on recent advances in the identification of downstream target genes and pathways.

Transcriptome of protoplasts reprogrammed into stem cells in Physcomitrella patens

Background

Differentiated plant cells can retain the capacity to be reprogrammed into pluripotent stem cells during regeneration. This capacity is associated with both cell cycle reactivation and acquisition of specific cellular characters. However, the molecular mechanisms underlying the reprogramming of protoplasts into stem cells remain largely unknown. Protoplasts of the moss Physcomitrella patens easily regenerate into protonema and therefore provide an ideal system to explore how differentiated cells can be reprogrammed to produce stem cells.

Principal findings

We obtained genome-wide digital gene expression tag profiles within the first three days of P. patens protoplast reprogramming. At four time-points during protoplast reprogramming, the transcript levels of 4827 genes changed more than four-fold and their expression correlated with the reprogramming phase. Gene ontology (GO) and pathway enrichment analysis of differentially expressed genes (DEGs) identified a set of significantly enriched GO terms and pathways, most of which were associated with photosynthesis, protein synthesis and stress responses. DEGs were grouped into six clusters that showed specific expression patterns using a K-means clustering algorithm. An investigation of function and expression patterns of genes identified a number of key candidate genes and pathways in early stages of protoplast reprogramming, which provided important clues to reveal the molecular mechanisms responsible for protoplast reprogramming.

Conclusions

We identified genes that show highly dynamic changes in expression during protoplast reprogramming into stem cells in P. patens. These genes are potential targets for further functional characterization and should be valuable for exploration of the mechanisms of stem cell reprogramming. In particular, our data provides evidence that protoplasts of P. patens are an ideal model system for elucidation of the molecular mechanisms underlying differentiated plant cell reprogramming.

Emerging roles of RETINOBLASTOMA-RELATED proteins in evolution and plant development

RETINOBLASTOMA-RELATED (RBR) proteins are plant homologs of the human tumor suppressor pRB. Similar to their animal counterparts they have roles in cell cycle regulation and differentiation. We discuss recent findings of the evolution of RBR functions ranging from a molecular ruler and metabolic integrator in algae to a coordinator of differentiation in gametophytes. Genetic analysis and manipulation of protein levels during gametophytic and post-embryonic plant development are now providing new insights into the function of RBR in stem cell maintenance, cell specification and differentiation. We briefly explain interactions of RBR with chromatin-modifying complexes that appear to be a central underlying molecular mechanism during developmental transitions.

Stomatal patterning and development

Stomata are epidermal pores used for water and gas exchange between a plant and the atmosphere. Both the entry of carbon dioxide for photosynthesis and the evaporation of water that drives transpiration and temperature regulation are modulated by the activities of stomata. Each stomatal pore is surrounded by two highly specialized cells called guard cells (GCs), and may also be associated with neighboring subsidiary cells; this entire unit is referred to as the stomatal complex. Generation of GCs requires stereotyped asymmetric and symmetric cell divisions, and the pattern of stomatal complexes in the epidermis follows a "one-cell-spacing rule" (one complex almost never touches another one). Both stomatal formation and patterning are highly regulated by a number of genetic components identified in the last decade, including, but not limited to, secreted peptide ligands, plasma membrane receptors and receptor-like kinases, a MAP kinase module, and a series of transcription factors. This review will elaborate on the current state of knowledge about components in signaling pathways required for cell fate and pattern, with emphasis on (1) a family of extracellular peptide ligands and their relationship to the TOO MANY MOUTHS receptor-like protein and/or members of the ERECTA receptor-like kinase family, (2) three tiers of a MAP kinase module and the kinases that confer novel regulatory effects in specific stomatal cell types, and (3) transcription factors that generate specific stomatal cell types and the regulatory mechanisms for modulating their activities. We will then consider two new proteins (BASL and PAN1, from Arabidopsis and maize, respectively) that regulate stomatal asymmetric divisions by establishing cell polarity.

WUS and STM-based reporter genes for studying meristem development in poplar

We describe the development of a reporter system for monitoring meristem initiation in poplar using promoters of poplar homologs to the meristem-active regulatory genes WUSCHEL (WUS) and SHOOTMERISTEMLESS (STM). When ~3 kb of the 5' flanking regions of close homologs were used to drive expression of the GUSPlus gene, 50-60% of the transgenic events showed expression in apical and axillary meristems. However, expression was also common in other organs, including in leaf veins (40 and 46% of WUS and STM transgenic events, respectively) and hydathodes (56% of WUS transgenic events). Histochemical GUS staining of explants during callogenesis and shoot regeneration using in vitro stems as explants showed that expression was detectable prior to visible shoot development, starting 3-15 days after explants were placed onto callus inducing medium. A minority of WUS and STM events also showed expression in the cambium, phloem, or xylem of regenerated, greenhouse grown plants undergoing secondary growth. Based on microarray gene expression data, a paralog of poplar WUS was detectably up-regulated during shoot initiation, but the other paralog was not. Both paralogs of poplar STM were down-regulated threefold to sixfold during early callus initiation. We identified 15-35 copies of cytokinin response regulator binding motifs (ARR1AT) and one copy of the auxin response element (AuxRE) in both promoters. Several of the events recovered may be useful for studying the process of primary and secondary meristem development, including treatments intended to stimulate meristem development to promote clonal propagation and genetic transformation.

Characterization of genes in the ASYMMETRIC LEAVES2/LATERAL ORGAN BOUNDARIES (AS2/LOB) family in Arabidopsis thaliana, and functional and molecular comparisons between AS2 and other family members

The ASYMMETRIC LEAVES2 (AS2) gene is required for the generation of the flat and symmetrical shape of the leaf lamina in Arabidopsis. AS2 encodes a plant-specific protein with an AS2/LATERAL ORGAN BOUNDARIES (AS2/LOB) domain that includes a cysteine repeat, a conserved single glycine residue and a leucine-zipper-like sequence in its amino-terminal half. The Arabidopsis genome contains 42 genes, including AS2, that encode proteins with an AS2/LOB domain in their amino-terminal halves, and these genes constitute the AS2/LOB gene family. In the present study, we cloned and characterized cDNAs that covered the putative coding regions of all members of this family, and investigated patterns of transcription systematically in Arabidopsis plants. Comparisons among amino acid sequences that had been deduced from the cloned cDNAs revealed eight groups of genes, with two or three members each, and high degrees of identity among entire amino acid sequences, suggesting that some members of the AS2/LOB family might have redundant function(s). Moreover, no member of the family exhibited significant similarity, in terms of the deduced amino acid sequence of the carboxy-terminal half, to AS2. Results of domain swapping between AS2 and other members of the family showed that the AS2/LOB domain of AS2 cannot be functionally replaced by those of other members of the family, and that only a few dissimilarities among respective amino acid residues of the AS2/LOB domain of AS2 and those of other members are important for the specific functions of AS2.

BAM receptors regulate stem cell specification and organ development through complex interactions with CLAVATA signaling

The CLAVATA1 (CLV1) receptor kinase regulates stem cell specification at shoot and flower meristems of Arabidopsis. Most clv1 alleles are dominant negative, and clv1 null alleles are weak in phenotype, suggesting additional receptors functioning in parallel. We have identified two such parallel receptors, BAM1 and BAM2. We show that the weak nature of the phenotype of clv1 null alleles is dependent on BAM activity, with bam clv mutants exhibiting severe defects in stem cell specification. Furthermore, BAM activity in the meristem depends on CLV2, which is required in part for CLV1 function. In addition, clv1 mutants enhance many of the Bam(-) organ phenotypes, indicating that, contrary to current understanding, CLV1 function is not specific to the meristem. CLV3 encodes a small, secreted peptide that acts as the ligand for CLV1. Mutations in clv3 lead to increased stem cell accumulation. Surprisingly, bam1 and bam2 mutants suppress the phenotype of clv3 mutants. We speculate that in addition to redundant function in the meristem center, BAM1 and BAM2 act to sequester CLV3-like ligands in the meristem flanks.

Asymmetric cell division--how flowering plant cells get their unique identity

A central question in biology is how cell fate is specified during development of a multicellular organism. Flowering plants use two major pathways of asymmetric cell divisions in a spatio-temporal manner to achieve required cellular differentiation. In the 'one mother--two different daughters' pathway, a mother cell mitotically divides to produce two daughter cells of different size and fate. By contrast, the 'coenocyte-cellularization' pathway involves formation of a coenocyte, nuclear migration to specific locations of the coenocyte and cellularization of these nuclei by unique wall forming processes. Given that cell fate determinants play a key role in establishing cell identity, their allocation to daughter cells in the two pathways needs to be understood in terms of the unique cell cycle regulatory mechanisms involved. Most of the information available on cell fate determination in flowering plants is in the form of genes identified from mutant analysis. Novel techniques of interrogating individual plant cells in vivo are necessary to advance the extant knowledge from genetics to functional genomics data bases.

Stem fasciated, a recessive mutation in sunflower (Helianthus annuus), alters plant morphology and auxin level

BACKGROUND AND AIMS

Plant lateral organs such as leaves arise from a group of initial cells within the flanks of the shoot apical meristem (SAM). Alterations in the initiation of lateral organs are often associated with changes in the dimension and arrangement of the SAM as well as with abnormal hormonal homeostasis. A mutation named stem fasciated (stf) that affects various aspects of plant development, including SAM shape and auxin level, was characterized in sunflower (Helianthus annuus).

METHODS

F1, F2 and F3 generations were obtained through reciprocal crosses between stf and normal plants. For the genetic analysis, a chi2 test was used. Phenotypic observations were made in field-grown and potted plants. A histological analysis of SAM, hypocotyl, epicotyl, stem and root apical meristem was also conducted. To evaluate the level of endogenous indole-3-acetic acid (IAA), a capillary gas chromatography-mass spectrometry-selected ion monitoring analysis was performed.

KEY RESULTS

stf is controlled by a single nuclear recessive gene. stf plants are characterized by a dramatically increased number of leaves and vascular bundles in the stem, as well as by a shortened plastochron and an altered phyllotaxis pattern. By histological analysis, it was demonstrated that the stf phenotype is related to an enlarged vegetative SAM. Microscopy analysis of the mutant's apex also revealed an abnormal enlargement of nuclei in both central and peripheral zones and a disorganized distribution of cells in the L2 layer of the central zone. The stf mutant showed a high endogenous free IAA level, whereas auxin perception appeared normal.

CONCLUSIONS

The observed phenotype and the high level of auxin detected in stf plants suggest that the STF gene is necessary for the proper initiation of primordia and for the establishment of a phyllotactic pattern through control of both SAM arrangement and hormonal homeostasis.

Plant stem cells carve their own niche

Stem cells are the precursors of differentiated cells and are, thus, indispensable for growth and development in plants and animals. Stem cells from both types of organisms share the fundamental features of a capacity for self-renewal and an ability to generate differentiated cells. The maintenance of stem cells in both systems is dependent upon reciprocal signalling between stem cells and the specialized tissue microenvironment known as the niche, which provides intercellular signals for stem cell regulation. One significantly underexplored facet of plant stem cells is the nature of their intrinsic transcriptional programme. A potentially rich avenue for addressing this deficiency is to combine laser-assisted microdissection and genome-wide transcriptional profiling to unravel the molecular road map controlling plant stem cells and their niches.

Nutrient sensing in plant meristems

Plants need nutrient to grow and plant cells need nutrient to divide. The meristems are the factories and cells that are left behind will expand and differentiate. However, meristems are not simple homogenous entities; cells in different parts of the meristem do different things. Positional cues operate that can fate cells into different tissue domains. However, founder/stem cells persist in specific locations within the meristem e.g. the quiescent centre of root apical meristem (RAM) and the lower half of the central zone of the shoot apical meristem (SAM). Given the complexity of meristems, do their cells simply respond to a diffusing gradient of photosynthate? This in turn begs the question, why do stem cell populations tend to have longer cell cycles than their immediate descendants given that like all other cells they are directly in the path of diffusing nutrient? In this review, we have examined the extent to which nutrient sensing might be operating in meristems. The scene is set for sugar sensing, the plant cell cycle, SAMs and RAMs. Special emphasis is given to the metabolic regulator, SnRK1 (SNF1-related protein kinase 1), hexokinase and the trehalose pathway in relation to sugar sensing. The unique plant cell cycle gene, cyclin-dependent kinase B1;1 may have evolved to be particularly responsive to sugar signalling pathways. Also, the homeobox gene, STIMPY, emerges strongly as a link between sugar sensing, plant cell proliferation and development. Flowering can be influenced by sucrose and glucose levels and both meristem identity and organ identity genes could well be differentially sensitive to sucrose and glucose signals. We also describe how meristems deal with extra photosynthate as a result of exposure to elevated CO2. What we review are numerous instances of how developmental processes can be affected by sugars/nutrients. However, given the scarcity of knowledge we are unable to provide uncontested links between nutrient sensing and specific activities in meristems.

Lower levels of transgene silencing in roots is associated with reduced DNA methylation levels at non-symmetrical sites but not at symmetrical sites

Transgene transcripts were recently shown to accumulate at higher levels in roots, relative to leaves, of silenced-transgenic Nicotiana benthamiana plants and to be inversely related with the accumulation of small interfering RNAs (siRNAs), suggesting that RNA silencing is less active in roots than in leaves (Andika et al., 2005. Mol. Plant-Microbe Interact. 18: 194). Here we show that the lower transgene RNA silencing activity in roots was associated with lower transgene methylation levels at non-symmetrical CpNpN context but not at symmetrical CpG or CpNpG context in three sets of transformant plants with different exogenous genes. In contrast, such a difference between roots and leaves was not observed for the Tnt1 retrotransposon: no Tnt1 transcript was detected in roots or in leaves of N. benthamiana, while equal levels of Tnt1-derived siRNA accumulation and Tnt1 methylation were found. From our data and previously reported information, we suggest that roots have less of an activity that acts at the step of generation of siRNAs.

Enhancing gene targeting efficiency in higher plants: rice is on the move

Meeting the challenge of routine gene targeting (GT) in higher plants is of crucial interest to researchers and plant breeders who are currently in need of a powerful tool to specifically modify a given locus in a genome. Higher plants have long been considered the last lineage resistant to targeting technology. However, a recent report described an efficient method of T-DNA-mediated targeted disruption of a non-selectable locus in rice [Terada et al., Nat Biotechnol 20: 1030-1034 (2002)]. Though this study was an obvious breakthrough, further improvement of GT frequencies may derive from a better understanding of the natural mechanisms that control homologous recombination (HR) processes. In this review, we will focus on what is known about HR and the factors which may hamper the development of routine GT by HR in higher plants. We will also present the current strategies envisaged to overcome these limitations, such as expression of recombination proteins and refinements in the design of the transformation vector.

Multicellularity, stem cells, and the neoblasts of the planarian Schmidtea mediterranea

All multicellular organisms depend on stem cells for their survival and perpetuation. Their central role in reproductive, embryonic, and post-embryonic processes, combined with their wide phylogenetic distribution in both the plant and animal kingdoms intimates that the emergence of stem cells may have been a prerequisite in the evolution of multicellular organisms. We present an evolutionary perspective on stem cells and extend this view to ascertain the value of current comparative studies on various invertebrate and vertebrate somatic and germ line stem cells. We suggest that somatic stem cells may be ancestral, with germ line stem cells being derived later in the evolution of multicellular organisms. We also propose that current studies of stem cell biology are likely to benefit from studying the somatic stem cells of simple metazoans. Here, we present the merits of neoblasts, a largely unexplored, yet experimentally accessible population of stem cells found in the planarian Schmidtea mediterranea. We introduce what we know about the neoblasts, and posit some of the questions that will need to be addressed in order to better resolve the relationship between planarian somatic stem cells and those found in other organisms, including humans.

Intercellular signalling in the transition from stem cells to organogenesis in meristems

Meristems continuously produce new cells to sustain plant growth. Stem cells are maintained in the centre of the meristem and provide the precursor cells for the initiation of new organs and tissues in the periphery. The structure of the meristem is maintained while cells are constantly displaced by new divisions. Recent advances have been made in understanding the intercellular signals that maintain meristem structure by adjusting gene expression according to cell position. In addition to refinements in our understanding of how the position and size of the stem-cell population is regulated, there have been advances in understanding how the location of new organ primordia is controlled and how the meristem influences organ polarity.

The evolution of plant development

The last decade has witnessed a resurgence in the study of the evolution of plant development, combining investigations in systematics, developmental morphology, molecular developmental genetics, and molecular evolution. The integration of phylogenetic studies, structural analyses of fossil and extant taxa, and molecular developmental genetic information allows the formulation of explicit and testable hypotheses for the evolution of morphological characters. These comprehensive approaches provide opportunities to dissect the evolution of major developmental transitions among land plants, including those associated with apical meristems, the origins of the root/shoot dichotomy, diversification of leaves, and origin and subsequent modification of flower structure. The evolution of these major developmental innovations is discussed within both phylogenetic and molecular genetic contexts. We conclude that it is the combination of these approaches that will lead to the greatest understanding of the evolution of plant development.